1. Field of the Invention
The present invention relates generally to lenses, and more particularly to lenses formed from a plurality of segments at least one of which has an axial refractive index gradient.
2. Prior Art
A conventional lens with spherical surfaces and with a homogeneous index of refraction will not focus light perfectly; there will be spherical and chromatic aberrations. The latter aberrations may be canceled, for example, by using a lens doublet in which each lens has a unique chemical composition and therefore an index of refraction with its own dependence on the wave length of light. The chromatic aberrations can be reduced by cancellation between the two indices. The spherical aberrations can be eliminated by the expensive and difficult process of grinding a predetermined aspherical surface on the lens. It is also well known in the art that these aberrations can be eliminated by employing axial gradient lens blanks. An axial gradient lens is a lens which has an index of refraction profile which varies in one direction only, usually chosen to be the optical axis. The spherical surface on these lenses provide the optical power. The axial gradient index profile which is exposed by the curving surface allows the index of refraction to vary along the surface. This variation can be chosen to minimize the spherical aberrations of the lens system. These lenses with reduced aberrations can be used advantageously in a variety of optical systems, such as slide projectors, cameras, binoculars, and many other imaging devices; the number of lens elements required for a given task can be reduced as well as the weight and complexity of the system.
The blanks for the fabrication of such gradient lenses can be made by a variety of processes such as SOL-GEL, infusion, and diffusion. In particular, there is the controlled diffusion process that can produce macro lenses with a prescribable index of refraction axial profile. The fabrication of such axial gradient lenses by the controlled diffusion process is disclosed in U.S. Pat. No. 5,262,896, "Refractive Elements With Graded Properties and Methods for Making Same", to R. Blankenbecler, which patent is incorporated herein by reference. These axial gradient lenses are commercially available.
Lens elements with a radial gradient index are also commercially available from Gradient Lens Corporation of Rochester N.Y., and from Nippon Sheet Glass Company, Ltd., Hyogo, Japan. Radial gradient lens elements in larger diameters is available from Progressive Photonics of Redwood Shores, Calif. These lens are symmetric around the optical axis and the transverse focal lengths are the same. They cannot be used to produce a symmetrical output beam from an unsymmetrical source such as a laser diode. An simple and improved type of lens with this capability is highly desirable.
An anamorphic lens is a lens that has different optical properties in the two directions, normally called the x- and y-axis, transverse to the optical axis, usually termed the z-axis. For example, the focal lengths in the x and y direction have different values. Such anamorphic lenses are useful in several contexts. One of the more important applications involves laser diode beam manipulation. The emitted light from a laser diode is more divergent in one transverse axis than in the other. The beam behaves in one direction as though it originates from a different depth in the source material than in the other direction, a property known as astigmatism. Thus an anamorphic lens system is required to focus this astigmatic beam into a parallel symmetric beam which is more useful.
A lens design for coupling a laser diode to a multimedia fiber using anamorphic radial gradient-index components is described by J. M. Stagaman and D. T. Moore, "Laser diode to fiber coupling using anamorphic gradient-index lenses", Applied Optics, vol. 23, no. 11, pp. 1730-1734 (1984). These authors discuss the disadvantages and difficulties in the conventional approach of utilizing prisms and/or cylindrical lenses. Their suggested design for a lens system to be used with a laser diode source with astigmatism uses a gradient index lens with an elliptical profile. At this time, there is no commercial method to fabricate a general anamorphic lens in which each transverse dimension has its own independent index profile. A new type of lens that allows the commercial fabrication of a general anamorphic lens is highly desirable.
A monolithic anamorphic lens fabricated from at least three segments with at least two possessing axial gradients is described in U.S. Pat. No. 5,541,774, issued Jul. 30, 1996 to R. Blankenbecler, the present author. In this patent, hereafter denoted by "the '774 patent", a more complete discussion of anamorphic lenses and prior art is given. The '774 patent is incorporated herein by reference.
Multiple element anamorphic lens systems can be formed by combining traditional cylindrical lenses with conventional lenses possessing radial symmetry. However high quality cylindrical lens elements are expensive to fabricate due to the difficulty of grinding and polishing accurate cylindrical surfaces. Thus a new design for an anamorphic lens that contains a minimum of segments and simple surfaces such as planer and spherical, is highly desirable.
It is therefore an object of the invention to provide an improved type of anamorphic lens that is a cemented monolithic structure to simplify mounting and adjustment.
It is another object of the invention to provide a general anamorphic lens in which each transverse dimension can has its own independent optical parameters which can be varied over a large range.
It is a related object of the invention to provide a general anamorphic lens system which provides independent manipulation of a beam of light in two transverse orthogonal directions.
It is a further object of the invention to provide a general anamorphic lens that does not require cylindrical surfaces to be ground and polished to a high precision.
It is a further object of the invention to provide a design for a general anamorphic lens that can be easily fabricated into a multiple lens array.